In AF (AutoFocus) devices for recent video cameras, the following focus adjustment method is a mainstream. That is, the sharpness of a frame is detected from a video signal obtained by photoelectrically converting an object image by an image sensing element or the like. The sharpness is set as an AF evaluation value, and movement of a focus lens is so controlled as to maximize the AF evaluation value.
In general, the AF evaluation value is the voltage level of the high-frequency component of a video signal extracted by a bandpass filter having a given band. When a normal object image is photographed, the voltage level (focus voltage level) of the high-frequency component of the video signal increases its voltage level value as the image is set in an in-focus state, as shown in FIG. 2. A point where the voltage level value maximizes is an in-focus position.
The hardware arrangement of an actual video camera will be described in detail with reference to FIG. 1.
In FIG. 1, reference numeral 101 denotes a fixed first lens group; 102, a zoom lens which performs zooming; 103, a stop; 104, a fixed second lens group; and 105, a focus compensation lens (to be referred to as a focus lens hereinafter) having a function of correcting movement of a focal plane along with zooming operation and a focusing function.
Reference numeral 106 denotes an image sensing element (CCD); 107, an AGC which amplifies an output from the CCD 106; and 108, a camera signal processing circuit which converts output signals from the AGC 107 into signals corresponding to a moving picture recording device 109 and still picture recording device 116 (to be described below). The moving picture recording device 109 uses a magnetic tape as a recording medium, and the still picture recording device 116 uses a semiconductor memory as a recording medium.
Reference numerals 110 and 112 denote motors for moving the zoom lens 102 and focus lens 105; 111 and 113, drivers which drive the motors 110 and 112 in accordance with signals from a camera AF microcomputer 115 (to be described below); and 114, an AF evaluation value processing circuit which extracts, from an output signal from the CCD 106, a high-frequency component and luminance difference component (difference between the maximum and minimum values of the luminance level of a video signal) used to perform focus detection.
The AF microcomputer 115 controls the motors 110 and 112 via the drivers 111 and 113 on the basis of output signals from the AF evaluation value processing circuit 114. The AF microcomputer 115 also performs various types of control such that the recording destination of an output signal from the camera signal processing circuit 108 is switched to the moving picture recording device 109 or still picture recording device 116 in accordance with ON operations of a moving picture trigger switch 117 and still picture release switch 118.
The AF microcomputer 115 comprises a CPU 115a, ROM 115b, and RAM 115c. The CPU 115a executes various processes on the basis of control programs (including control programs corresponding to flow charts (to be described later)) stored in the ROM 115b. At this time, the CPU 115a uses the RAM 115c as a work area or the like.
In the camera system having the arrangement shown in FIG. 1, the AF microcomputer 115 automatically adjusts the focus by moving the focus lens 105 so as to maximize the output signal level of the AF evaluation value processing circuit 114. The AF microcomputer 115 issues a recording instruction to the moving picture recording device 109 upon reception of a moving picture trigger signal upon ON operation of the moving picture trigger switch 117. The AF microcomputer 115 issues a recording instruction to the still picture recording device 116 upon reception of a release signal upon ON operation of the still picture release switch 118.
AF control by the AF microcomputer 115 in photographing a moving picture will be explained in detail with reference to FIGS. 3 to 7.
After moving picture AF processing starts (step S301), the CPU 115a of the AF microcomputer 115 finely drives the focus lens 105 (step S302). Fine driving processing will be described in detail later with reference to FIG. 4. Then, the CPU 115a checks whether the focus lens 105 is in focus by fine driving (step S303). If NO in step S303, the CPU 115a checks whether the in-focus direction is determined by fine driving (step S304).
If NO in step S304, the CPU 115a returns to step S302. If YES in step S304, the CPU 115a advances to step S305, and performs so-called hill-climbing driving of moving the focus lens 105 at a high speed in a direction in which the AF evaluation value increases. Hill-climbing driving processing will be described in detail later with reference to FIG. 6. The CPU 115a checks whether the AF evaluation value exceeds its peak by hill-climbing driving (step S306). If NO in step S306, the CPU 115a returns to step S305, and continues hill-climbing driving.
If YES in step S306, the CPU 115a drives the focus lens 105 in an opposite direction in order to return the AF evaluation value to its peak during hill-climbing driving (step S307). Then, the CPU 115a checks whether the AF evaluation value reaches its peak (step S308). If NO in step S308, the CPU 115a returns to step S307, and continues the operation of returning the AF evaluation value to its peak. If YES in step S308, the CPU 115a returns to step S302, finely drives the focus lens 105, and searches for the in-focus position of the next moving picture.
If YES in step S303, the CPU 115a stores an AF evaluation value for an in-focus state in the RAM 115c (step S309), and performs reactivation determination processing for moving picture AF operation (step S310). In reactivation determination processing, the CPU 115a compares the current AF evaluation value stored in step S309 with the previous AF evaluation value, and if the values are different by a predetermined level or more, the CPU 115a determines that the focus lens 105 must be reactivated.
The CPU 115a checks whether the focus lens 105 is determined to be reactivated in reactivation determination processing (step S311). If YES in step S311, the CPU 115a returns to step S302, and restarts fine driving operation in order to execute AF processing for the next moving picture. If NO in step S311, the CPU 115a stops the focus lens 105 (step S312). The CPU 115a returns to step S310 in order to perform AF control for subsequent moving pictures, and continues reactivation determination processing.
Details of fine driving processing in step S302 of FIG. 3 will be explained with reference to the flow chart of FIG. 4, and FIG. 5.
After fine driving processing starts (step S401), the CPU 115a receives an AF evaluation value from the AF evaluation value processing circuit 114 (step S402). The CPU 115a checks whether the current AF evaluation value received in step S402 is larger than the previous AF evaluation value (step S403).
If NO in step S403, the CPU 115a advances to step S404, and moves the focus lens 105 by a predetermined amount in an opposite direction. If YES in step S403, the CPU 115a advances to step S405, and moves the focus lens 105 by a predetermined amount in the current direction (forward/backward direction).
After the process in step S404 or S405, the CPU 115a checks whether the direction determined as an in-focus direction is kept unchanged successively a predetermined number of times or more, i.e., whether the focus lens 105 moves in the same direction successively a predetermined number of times or more (step S406).
If YES in step S406, the CPU 115a sets that the moving direction of the focus lens 105 for an in-focus state can be determined (step S407), and ends fine driving processing. When fine driving processing ends through this route, hill-climbing driving in step S305 of FIG. 3 is executed.
If NO in step S406, the CPU 115a checks whether the focus lens 105 repeats direction reversal a predetermined number of times or more in almost the same area (step S408). If NO in step S408, this means that the focus lens 105 has not reached the vicinity of an in-focus position. The CPU 115a returns to step S402, and continues fine driving processing.
If YES in step S408, this means that the focus lens 105 has reached the vicinity of an in-focus position (step S409), and the CPU 115a ends fine driving processing. When fine driving processing ends through this route, the reactivation determination routine in step S310 of FIG. 3 is executed.
The processes in steps S403 to S405 will be described with reference to FIG. 5.
In FIG. 5, the CPU 115a receives at a timing TA an AF evaluation value A for charges (image signal) accumulated in the CCD 106 during a period A, and receives at a timing TB an AF evaluation value B for an image signal accumulated in the CCD 106 during a period B. At the timing TB, the CPU 115a compares the AF evaluation values A and B, if A<B holds, keeps moving the focus lens 105 in the forward direction (current direction), and if A>B holds, moves the focus lens 105 in an opposite direction.
Details of hill-climbing driving processing in step S305 of FIG. 3 will be explained with reference to the flow chart of FIG. 6, and FIG. 7.
After hill-climbing processing starts (step S601), the CPU 115a receives an AF evaluation value from the AF evaluation value processing circuit 114 (step S602). The CPU 115a checks whether the current AF evaluation value received in step S602 is larger than the previous AF evaluation value (step S603).
If YES in step S603, the CPU 115a drives the focus lens 105 in the forward direction at a predetermined speed (step S604), and returns to step S602.
If NO in step S603, the CPU 115a checks whether the AF evaluation value exceeds its peak (step S605). If the AF evaluation value does not exceed its peak, i.e., the current AF evaluation value becomes equal to or smaller than the previous AF evaluation value though the AF evaluation value does not exceed its peak, the AF microcomputer 115 determines that the direction is not correct, drives the focus lens 105 in an opposite direction at a predetermined speed (step S606), and returns to step S602.
If the AF evaluation value exceeds its peak, i.e., the current AF evaluation value becomes equal to or smaller than the previous AF evaluation value as a result of exceeding the peak of the AF evaluation value, the CPU 115a determines that an in-focus point exists, and ends hill-climbing driving processing (step S607). When hill-climbing processing ends in this manner, fine driving processing is executed in step S302 of FIG. 3.
The significance of the processes in steps S605 to S607 of FIG. 6 will be supplemented with reference to FIG. 7.
In FIG. 7, the upper MA decreases the AF evaluation value over the peak. The CPU 115a determines that an in-focus point exists and the focus lens 105 has passed through the in-focus point. Thus, the CPU 115a ends hill-climbing operation, and shifts to fine driving processing. The lower MB decreases the AF evaluation value without any peak. The CPU 115a determines that the moving direction of the focus lens 105 is not correct, reverses the moving direction, and continues hill-climbing operation.
As described above, the camera AF microcomputer 115 always maximizes the AF evaluation value by controlling movement of the focus lens 105 while repeating reactivation determination→fine driving→hill-climbing driving→fine driving→reactivation determination.
Japanese Patent Laid-Open No. 07-298120 proposes a method of normalizing an AF evaluation value by a luminance difference component and determining an in-focus degree. This method exploits the fact that the ratio of the luminance difference component and a high-frequency component serving as an AF evaluation value is constant at an in-focus point. If the ratio is a predetermined value or more, the focus lens is close to an in-focus point. If the ratio is very low, the focus lens is greatly in an out-of-focus state. In other words, an in-focus state can be determined to a certain extent from the ratio of the maximum value of the luminance difference component and the AF evaluation value. The determination result is used to tune the amplitude in fine driving (vibrations or reciprocation) or the speed in hill-climbing driving.
In recent years, video cameras having a still picture photographing mode have been implemented. In AF processing in photographing a still picture by this video camera, the focus lens 105 is moved in accordance with release operation for still picture photography to a lens position corresponding to the maximum AF evaluation value which has already been obtained by moving picture AF processing. Alternatively, in-focus control is performed again.
The former conventional AF processing in still picture photography will be explained with reference to the flow chart of FIG. 16.
After AF processing starts (step S1601), the CPU 115a of the AF microcomputer 115 executes AF processing in moving picture photography that has been described with reference to FIGS. 3 to 7 (step S1602). The CPU 115a checks whether the still picture release switch 118 has been turned on to input a still picture release signal (step S1603). If NO in step S1603, the CPU 115a returns to step S1602, and continues AF processing in moving picture photography.
If YES in step S1603, the CPU 115a moves the focus lens 105 to a position corresponding to the maximum AF evaluation value obtained by the preceding AF processing in moving picture photography (step S1604). The CPU 115a records a still picture by controlling the camera signal processing circuit 108 and still picture recording device 116 (step S1605), and ends AF processing in still picture photography (step S1606).
The latter conventional AF processing in still picture photography will be described with reference to the flow chart of FIG. 17.
After AF processing starts (step S1701), the CPU 115a of the AF microcomputer 115 executes AF processing in moving picture photography that has been described with reference to FIGS. 3 to 7 (step S1702). The CPU 115a checks whether the still picture release switch 118 has been turned on to input a still picture release signal (step S1703). If NO in step S1703, the CPU 115a returns to step S1702, and continues AF processing in moving picture photography.
If YES in step S1703, the CPU 115a moves the focus lens 105 to the closest focusing (wide-angle) direction at a high speed (step S1704), and checks whether the AF evaluation value decreases (step S1705). If NO in step S1705, the CPU 115a returns to step S1704, and continues lens moving processing to the closest focusing direction.
If YES in step S1705, the CPU 115a moves the focus lens 105 to the infinity (telephoto) direction at a high speed (step S1706). The CPU 115a monitors changes in AF evaluation value, and checks whether the AF evaluation value exceeds its peak (step S1707). If NO in step S1707, the CPU 115a returns to step S1706, and continues lens moving processing to the infinity direction.
If YES in step S1707, the CPU 115a moves the focus lens 105 to the peak position (in-focus position) (step S1708). The CPU 115a performs fine driving in FIG. 4 to search for an accurate peak position (step S1709). Fine driving processing is done in consideration of a case in which an actual in-focus position includes an error even if a peak position is detected during high-speed driving, or a case in which an object to be photographed moves.
The CPU 115a checks whether a peak position has been detected by fine driving processing of step S1709 (step S1710). If NO in step S1710, the CPU 115a returns to step S1709, and repeats fine driving processing.
If YES in step S1710, the CPU 115a moves the focus lens 105 to the peak position (step S1711). The CPU 115a records a still picture by controlling the camera signal processing circuit 108 and still picture recording device 116 (step S1712), and ends AF processing in still picture photography (step S1713).
This prior art suffers the following problems. The moving time is short when the focus lens 105 is moved in accordance with release operation for still picture photography to a lens position corresponding to the maximum AF evaluation value obtained by the preceding moving picture AF processing. If the lens position corresponding to the maximum AF evaluation value obtained by moving picture AF processing is not an in-focus position, a blurred still picture is captured.
When in-focus control is newly executed though the lens position corresponding to the maximum AF evaluation value obtained by moving picture AF processing is an in-focus position, a predetermined time is necessarily required till reception of an image, undesirably generating a shutter time lag.